In analog television systems, moving video pictures are transmitted as analog signals, continuous in time and amplitude, over a carrier waveform. A moving picture is typically transmitted as a sequential set of “fields”, each describing a static image for a receiver device to render on a screen. Each static image for the receiver device to render is substantially “flat”. In other words, the static image is merely a collection of pixels without any meaning. A pixel is the smallest unit of an image that can be represented or controlled.
In the analog world, there is no concept of layering objects in video pictures, as the receiver device is not operative to manipulate such objects. The generation of objects to be displayed as an overlay on a video picture is done at the transmitting device, which integrates the objects into the video picture. The receiver device in the analog domain passively displays these objects and does not have technical capability to alter them in any fashion. For example, with analog video broadcast signals, there is a mechanism called closed captioning to transmit a transcription of the audio portion of a program as it occurs, sometimes including non-speech elements. Video receivers enabled to decode and display the contents of closed captions typically allow the viewer to toggle on/off the display of the closed captions. However, the positioning of an object, such as a closed captioning window, on the video picture typically is not under the control of the viewer or the receiver device. Rather, said positioning is encoded at the source. As a result, especially in live broadcasts, the closed captioning window or other objects are often integrated within areas of the video picture that are desirable for viewing, thereby occluding (blocking) the same.
In addition, within the realm of analog video broadcasts exists the concept of Picture-in-Picture (PiP). This feature typically allows displaying one program on the full screen while one or more other programs are displayed in inset windows. PiP requires two independent tuners or signal sources. The receiver device displays the output of one tuner in full screen and the secondary tuner's output as an overlaid picture on top of the primary picture in the full screen. In this case, the receiver device typically enables the viewer to select the position of the PiP window within a set of pre-selected positions on the screen.
In the digital world, video pictures are transmitted as a stream of data within the frame of a CODEC (COder-DECoder). A CODEC is a device or computer program capable of encoding and/or decoding a digital video stream or signal. A CODEC is able to interpret instructions within the video stream and present a displayed object. Some CODECs support the embedding of one secondary video stream within a primary video stream. In this case, the embedding of a secondary video stream, which contains an object to be displayed within a frame of the primary display (video stream), typically contains position information for the object to be displayed. However, the part of the screen that is displaying a primary video stream and is located beneath the object is occluded and thus not visible to the viewer. This degrades the viewer's ability to experience the fullness of the video stream.